High-strength austenitic stainless steel for a boiler



Get. 13, 1970.

EU! ,mvbsl-u E All HIGH-STRENGTHKUSTENITIG STAINLESS STEEL FOR A BOILER Filed Feb. 5, 1967 FIG. 3A

FIG. 30

2 Sheets-Sheet 2 INVENTORE Eldl MIYOSHI y TERUO YuKn'osH/ HTTORNEYS United States Patent 3,533,780 HIGH-STRENGTH AUSTENITIC STAINLESS STEEL FOR A BOILER Eiji Miyoshi, Hyogo-ken, and Teruo Yukitoshi, Osakafu, Japan, assignors to Sumitomo Kinzoku Kogyo Kabushiki Kaisha, Osaka-shi, Japan Filed Feb. 3, 1967, Ser. No. 613,894 Int. Cl. C22c 39/20 US. Cl. 75128 8 Claims ABSTRACT OF THE DISCLOSURE An austenitic stainless steel having high creep rupture strength and corrosion resistance at high temperatures and useful for making boiler tubes and the like compromises 0.01-0.20% carbon, 0.11.0% silicon, 0.14.0% manganese, ODS-0.15% phosphor, 12-18% chromium, 12-20% nickel, -25% molybdenum, 0.12.0% niobium, 0.0010.05% boron and the balance iron plus residual impurities.

This invention relates to an austenitic stainless steel useful as a material for the boiler tubes at high temperature and pressure, especially for those of a super-heater attached to a super-critical pressure type boiler.

In general such tubes are required to meet the following requirements:

(1) high creep rupture strength,

(2) stability of the tube material throughout long time use,

(3) good weldability,

(4) good corrosion and oxidation resistance and (5) good bend workability.

The austenitic stainless steel of this invention comprises 0.010.20% carbon, O.11.0% silicon, 0.14.0'% manganese, ODS-0.15% phosphor, 12-18% chromium, 12-20% nickel, 0.52.5% molybdenum, 0.12.0% niobium, 0.0010.05% boron, balance iron except incidental nitrogen and impurities.

Conventional austenitic heat resisting steels for a boiler which have heretofore been generally used include a 188 type austenitic steel, ANSI, 17-14 Cu-Mo, l5N, Esshete 1250 steel and the like. And other steels such as G18B, N155 steel, etc. are considered also usable for this purpose. Said conventional austenitic steels, however, have lower high-temperature creep rupture strength than the steel of this invention; and while having a high strength celerating precipitation with the addition of phosphorus, and that the toughness and creep rupture strength at elevated temperature thereof have been improved by the addition of boron. Thus said steel will have therein lot of precipitates such as carbide and nitride because the step of precipitation hardening is taken in the preparation of the steel and especially nitrogen is added thereto, and therefore this will make it undesirable to use the steel as a material for boiler tubes and will cause the corrosion resistance of the steel to lower remarkably. In addition, because said steel have lower workability for tube making and bending and lower weldability due to its high hardness, it can be a desirable material for valves but is an unsuitable one for boiler tubes. It has thus been hoped to r produce a steel material for boiler tubes, which meets various requirements for the tubes and has further improved creep rupture strength at elevated temperature.

It has been discovered after many years of our study that there can be produced an alloyed steel which has a completely austenitic structure, qualitative stability with little change such as precipitation, etc. even when in use over a long period of time, high creep rupture strength and low price.

The steel of this invention is prepared from a heatresisting austenitic steel containing chromium, nickel,

molybdenum, and some other additives hereinafter mentioned by adding niobium content corresponding to the carbon content to prohibit the precipitation of chromium carbide whereby the starting steel is improved mainly in resistance against intercrystalline corrosion and that in high temperature strength, moreover, this steel has great increase in high-temperature strength by further adding to the niobium-added steel, boron in a very small amount together with phosphorus in a suitable amount to effect solid solution strengthening.

As is seen from the foregoing, the steel of this invention is characterized in that it durably retains its high-temperature strength without resort to precipitation effect which he said austenitic steel for valve has resorted to.

FIG. 1 shows rupture stress-time curves for the steels of this invention and of conventional type;

FIG. 2 shows the effect of each element added together with boron on the creep rupture strength; and

FIGS. 3A and 3B shows enlarged microstructures of steels A and B respectively of the present invention.

FIG. 3C shows an enlarged microstructure of steel C which is a conventional steel.

The steels A and B of this invention as well as the steels C, D, E, F and G of conventional type listed in at elevated temperature, other special alloy steels cono tain expensive alloying elements such as cobalt, etc. in Tabl? 1 were each tested at 7 for determfnmg the l i l large amounts whereby a problem of cost is relation between stress applied and rupture time, and i d the results obtained are graphically shown in FIG. 1. In There has been also proposed another austentic steel addition, Table 2 indicates the Creep rupture strengths of for valves. In said austenitic steel for valves, the highall the Steels llsted in Table 1 after having been Subjected temperature strength thereof has been enhanced by acto 700 C. for 10,000 hours.

' TABLE 1 Element Steel 0 Si Mn P Cr Ni M0 Nb B1 V Zr N W Go Remarks Steels of this invention:

A 0.59 2.80 0.092 13.89 13.72 1.56 0.45 0.50 2.72 0.093 15.74 14.02 1.62 1.17 0.51 1.46 0.144 13.63 16.47 1.47 0.94 0.0080.012B10.05-0.15 P. 0.55 1.43 0. 091 13. 50 16.21 1.38 0.98 0.015 More than 0.01 B 0.05-0.15 P. 0. 52 1. 45 0.132 13.48 16.3 1.42 1. 03 0.014 More than 0.012 B 0.12-0.15 P. 0.53 1.41 0.081 13.34 16.10 1.35 1.61 0.10-0.20 o. 0.55 1.45 0.094 13.80 16.50 2.10 0.87 0.0 2.0-2.5Mo. 0.50 1.47 0.088 13.75 16.62 1.35 1.10 0.74 2.87 0.010 19.67 13.89 1.53 0.95

l Boron.

3 TABLE 2 Creep rupture strength of hrs. (kg/mm?) at 700 C.

As is apparent from FIG. 1 and Table 2, the steels of this invention show a very superior creep rupture strength of at least 14.8 kg./mm. after having been subjected to a temperature of 700 C. for 10,000 hours.

Table 3 shows not only the Charpy impact values for the steels A and B of this invention and the steels C and D of conventional type listed in Table 1 after solid solution heat treatment only, but also those for the same steels after having been heated to 750 C. for 3,000 hours and then quenched with water, in addition to said solid solution heat treatment. The Charpy impact values shown in Table 3 are considered a criterion according to which the future structure stability of the steels when hereafter used over a long period of time will be anticipated, and this implies the steels of this invention have a superior property such as temperature and time stability.

Charpy impact value (kg.m./em.

There will be hereinafter fully mentioned the reason why the amounts of the additive components to be used in the production of the steels of this invention should be restricted to within certain ranges, respectively.

FIG. 2 shows the effect of the specific element, Zr, V, Mn, P or N, when having been added together with B (boron) to the Cr-Ni-Mo-Nb steel as the base steel for this invention on the creep rupture strength of the base steel containing boron. In FIG. 2, the abscissa represents the amounts of the specific elements added together with boron to the base steel, and the ordinate represents the ratio of the creep rupture time at 700 C. and 18 kg./mm. for the base steel containing boron and one of the specific elements to that for the base steel containing boron and none of the elements.

As is apparent from FIG. 2, phosphorus is the only element which will be useful for the increase of creep rupture strength when added together with boron, and the effect of the simultaneous addition of boron and phosphorus on such strength will be great; and on the other hand the other elements will each have an adverse effect such that the creep rupture strength is lowered, and this is particularly the case with nitrogen. The steels of this invention are thus importantly characterized in that they contain no nitrogen.

Boron is generally effective for increasing the creep strength of the base steel when added thereto in a very small amount, while it is less effective when added in an amount less than 0.001% and lowers the weldability of the steel when added in an amount of more than 0.05%. This shows that the addition of boron in amounts mentioned above is not desirable. In addition, the addition of boron in an amount of less than 0.005% is little effective for increasing the high-temperature toughness, and from the viewpoint of weldability, it is preferred that the amount of boron be less than 0.012%. Phosphorus is mainly dissolved in a matrix to form a solid solution therewith whereby the matrix increases in strength. Phosphorus is, however, less effective for increasing the strength when added in an amount of less than 0.05%; and it promotes the precipitation effect of chromium carbide, deteriorates the plastic workability, makes tube production work difficult and impairs the bend workability, when added in an amount of more than 0.15%. And the addition of phosphorus even in an amount of more than 0.12% deteriorates the corrosion resistance under stress.

Carbon has effects on the properties of steels by forming carbide, and a high carbon steel is satisfactory where a steel having increased short-time tensile strength is required. Carbon, however, increases the precipitation of carbide thereby to make brittle the steel to which the carbon has been added and lower the corrosion resistance thereof, when added in an amount of more than 0.20%. A low carbon steel is accordingly desirable as a steel having a compositional stability with formation of less carbide when in use for a long time, while such a low carbon steel the carbon content of which is less than 0.01% has unsatisfactory strength and is difficult to produce prac tically. The addition of carbon in an amount of more than 0.1 produces a steel in which carbide is liable to precipitate and the steel structure is also liable to be unstable.

Niobium is effective for not only prohibiting the precipitation of chromium carbide but also greatly increasing the high-temperature strength, and the amount thereof to be added depends upon that of the carbon contained in the matrix and it should thus be ten times the amount of the carbon, that is, O.12.0% in this invention. However, it is to be noted that the addition of niobium in an amount of more than 1.5% causes inter-metallic compounds to be liable to form thereby to make unstable the structure of the steel to which the niobium has been added.

Molybdenum, as a Solute atom, is effective for enhancing the strength. It is, however, less effective for this purpose when added in an amount of less than 0.5%; and it promotes the formation of ferrite and forms intermetallic compounds with other metallic elements thereby to make brittle the steel containing it if heated to a high temperature for a long time, when added in an amount of more than 2.5%. Such brittleness is, of course, undesirable. In addition, intermetallic compounds are made liable to form in the steel and the structure of the steel is made liable to be unstable when molybdenum has been added to the steel an an amount of more than 2.0%.

The addition of more than 16% nickel does not increase the high-temperature strength for the increase of production cost due to the addition.

The addition of less than 0.01% zirconium and that of less than 0.1% Vanadium are respectively little effective for increasing high-temperature strength and toughness. And there is obtained little effect for the increase of production cost when more than 0.05% zirconium or more than 0.5% vanadium has been added.

In FIGS. 3A, 3B, 30 there are shown the microstructure, in magnified form (X500), of the structure of steels prepared by treating the steels A and B and the similar steel C listed in Table 1 at 1,200 C. for an hour to form them into solid solutions, thereafter heating the treated steels at 750 C. for 3,000 hours and then quenching them with water. FIGS. 3A, 3B, 3C show the steels thus prepared from the steels A, B and C, respectively.

As previously mentioned, it is required that steel material for boiler tubes be still stable in structure with toughness being little lowered after having been in use at a high temperature for a long period of time. The steel material of this invention will meet such requirements as mentioned below. As is apparent from FIGS. 3A, 3B and 30 the steel material of this invention is not greatly different in structure from the same material prior to the long-term heating and this proves the material to be substantially stable in structure; and the steel A containing 0.02% carbon shown in FIG. 3A is particularly noted to have less precipitates therein and satisfactory toughness represented by Charpy impact value of 8-10 kg.-rn./cm. from Table 3. On the other hand, the steel C which is similar stee at FIG. 3C shows its structure wherein a great deal of intermetallic compounds has been precipitated on the grain boundaries and within the grains, and shows remarkably decreased toughness represented by Charpy impact value of less than 1 kg.-m./cm. as seen from Table 3. It is understoood that the steel C, which had been perfectly austenitic in solid solution state treatment, caused the precipitate of intermetallic compound such as a-phase by the long-time heating because the steel containing 20% chromium and 14% nickel is, in composition, in proximity to ferrite.

The steel accordin to this invention, therefore, should contain chromium and nickel in amounts of 12-18% and 12-20%, respectively, to assure that it has corrosion resistance, oxidation resistance, and toughness substantially unchanged after its long-time use. Said steel should also contain silicon as a deoxidation element in an amount of 01-10% which can be added without lowering the strength of the steel, and further contain manganese in an amount of 0.1-4.0% for using as a deoxidization element similar to the silicon and improving the steel in workability. In this connection, the use of manganese in an amount of more than 4% should be avoided because it will remarkably lower the strength of the steel.

As previously described, the steels according to this invention, which are useful not only as a material for boiler tubes but also as a material where high creep rupture strength and corrosion resistance are required at a high temperature, can be produced without using expensive alloying elements at relatively low cost; and the use of these steels will make it possible to manufacture therefrom boiler tubes having a Wall of smaller thickness whereby the tubes can be made lighter in weight as well as lower in price and higher temperature and pressure type boilers can be designed and realized. This will make great contributions to the designing of proposed super-critical pressure type boilers.

What is claimed is:

1. An austenitic stainless steel useful as a material for boiler tubes which has higher creep rupture strength at elevated temperature and a structure still stable after in use at a high-temperature for a long time, consisting essentially of 0.01-0.20% carbon, 0.1-1.0% silicon,

6 01-40% manganese, ODS-0.15% phosphorus, 0.001- 005% boron, 12-18% chromium, 12-20% nickel,

05-25% molybdenum, 0.1-2.0% niobium, balance iron plus residual impurities.

2. A steel as claimed in claim 1 wherein boron is contained in an amount of 0.005-0.012% thereby to improve weldability and high-temperature toughness.

3. A steel as claimed in claim 1 wherein phosphorus is contained in an amount of ODS-0.12% thereby to improve resistance against corrosion under stress.

4. A steel as claimed in claim 1 wherein niobium is contained in an amount of 0.1-1.5 thereby to improve resistance against intercrystalline corrosion.

5. A steel as claimed in claim 1 wherein phosphorus, boron and niobium are contained in respective amounts of 0.05-0.12%, 0.005-0.012% and 0.1-1.5% thereby to improve weldability, resistance against corrosion under stress, resistance against intercrystalline corrosion and high-temperature toughness.

6. A steel as claimed in claim 1 wherein carbon is contained in an amount of 0.01-0.1% thereby to improve structure stability while in use over a long period of time.

7. A steel as claimed in claim 5 wherein carbon, phosphorus, nickel and molybdenum are contained in amounts of 0.01-0.1%, ODS-0.12%, 12-16%. and 05-20%, respectively, thereby to improve the same properties as claim 5.

8. An austenitic stainless steel useful as a material for boiler tubes which has higher creep rupture strength at elevated temperature and a structure still stable after in use at a high temperature for a long time, consisting essentially of 0.01-0.20% carbon, 0.1-1.0% silicon, 0.1-4.0% manganese, 0.-05-0.l5% phosphorus, 0.001- 0.05% boron, 12-18% chromium, 12-20% nickel, 05-25% molybdenum, 0.1-2.0% niobium, at least one metal selected from the group consisting of zirconium and vanadium in respective amounts of 0.01-0.05% and 01-05%, and the balance iron plus residual impurities.

References Cited UNITED STATES PATENTS 2,111,278 3/ 1938 Charlton. 2,880,085 3/1959 Kirkby -128.6 X 2,159,724 5/1939 Franks 75l28.6 X 2,984,563 3/1961 Tanczyn 75l28.9 X 3,303,023 2/ 1967 Dulis.

HYLAND BIZOT, Primary Examiner 

